Track if invoices have been sent via email (#2231)

src/pretix/base/migrations/0198_invoice_sent_to_customer.py

@@ -0,0 +1,21 @@
+# Generated by Django 3.2.4 on 2021-09-30 10:25
+from datetime import datetime
+
+from django.db import migrations, models
+from pytz import UTC
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('pretixbase', '0197_auto_20210914_0814'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='invoice',
+            name='sent_to_customer',
+            field=models.DateTimeField(blank=True, null=True, default=UTC.localize(datetime(1970, 1, 1, 0, 0, 0, 0))),
+            preserve_default=False,
+        ),
+    ]